Digital ultrasonic densitometer

ABSTRACT

An ultrasonic instrument employs a digital architecture to provide improved stability to sound speed measurements of human bone in vivo. A digitization of the received ultrasonic signal allows numerical analyses to be applied in determining precise arrival time of the waveform. The microprocessor may initiate the ultrasonic signal transmission and detect its arrival and may control the strength of the transmitted signal and the amplification of the received signal to optimize the signal path.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of Ser. No. 09/277,481 filed Mar. 26,1999, which claims benefit to Ser. No. 60/080,158 Mar. 31, 1998; U.S.Pat. No. 6,277,076

and is a CIP of Ser. No. 09/094,073 Jun. 9, 1998 U.S. Pat. No.6,027,449;

which is a CIP of Ser. No. 08/795,025 Feb. 4, 1997 Nov. 24, 1998 U.S.Pat. No. 5,840,029;

which is a CIP of Ser. No. 08/466,495 Jun. 6, 1995 Feb. 18, 1997 U.S.Pat. No. 5,603,325;

which is a CIP of Ser. No. 08/397,027 Mar. 1, 1995 Jan. 16, 1996 U.S.Pat. No. 5,483,965;

which is a CIP of Ser. No. 08/072,799 Jun. 4, 1993 Abandoned;

which is a CIP of Ser. No. 07/895,494 Jun. 8, 1992 Sep. 6, 1994 U.S.Pat. No. 5,343,863;

which is a CIP of Ser. No. 07/772,982 Oct. 7, 1991 Jun. 9, 1992 U.S.Pat. No. 5,119,820;

which is a CIP of Ser. No. 07/343,170 Apr. 25, 1989 Oct. 8, 1991 U.S.Pat. No. 5,054,490;

which is a CIP of Ser. No. 07/193,295 May 11, 1988 Jun. 5, 1990 U.S.Pat. No. 4,930,511.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to devices which are used for measuringthe density of members, such as bones, and more particularly to deviceswhich utilize ultrasonic acoustic signals to measure the physicalproperties and integrity of the members.

2. Description of the Prior Art

The present invention relates to bone measuring instruments and inparticular to instruments employing ultrasonic pulses to makemeasurements of the integrity of human bones in vivo.

Osteoporosis or loss of bone mineralization and its cure or preventionare important areas of medical and biological interest. Of particularconcern is the loss of trabecular bone, a spongy bone structure formingthe interior of vertebrae and other bones. The trabecular bone providesmuch of the strength of such bones and is disproportionately affected intimes of bone mass loss. While it has long been known that the speed ofsound through a material will reveal properties of that material,application of this principle for reliable clinically significant bonedensity measurement has not been easy.

An early measurement of sound speed through in vitro bone is describedin: Sonic Measurement of Bone Mass, Clayton Rich et al., a paperdelivered at a conference held in Washington, D. C. Mar. 25-27, 1965,NASA publication SP-64. A sinusoidal pulse was timed in its passage froma transmitting transducer through a water bath and excised bone to areceiving transducer. The author noted problems of providing sufficientsignal strength in trabecular bone even with the use of an automaticgain control circuit.

Sound speed measurement of bone in vivo is described in U.S. Pat. No.3,847,141 to Hoop issued Nov. 12, 1974. A pulse from a transmittingtransducer was propagated through a finger to be received by a receivingtransducer; however, the pulse was not timed directly. Rather afterfiltering, the pulse was used to “retrigger” the transmitting transducerto create a regular series of pulses whose frequency could bedetermined. This technique is known generally as “sing around”.

A doctoral thesis by Langton entitled “The Measurement of BroadbandUltrasonic Attenuation in Cancellous Bone” dated July 1984 describes themeasurement of the speed of sound through the os calcis of the heel. Theauthor, however, found that accurate measurement of sound speed washampered by the difficulty of measuring the beginning of the pulse on anoscilloscope and suggested that the elapsed time between thetransmission and reception of a pulse was too short for accurate andrepeatable measurements. He proposed that the “sing around” approach ofHoop might be used to correct this latter problem. Langton, observingthe considerable frequency distortion of the pulse after passage throughthe heel, elected to continue his investigation in the area of frequencydependant attenuation rather than sound speed.

BRIEF SUMMARY OF THE INVENTION

The present invention provides significantly improved accuracy in speedof sound measurements of bone in vivo. The present inventors haverecognized that prior-art detection systems, using analog thresholddetection techniques, were susceptible to timing variations caused byphase and amplitude distortion of the transmitted pulse. Even smallvariations in detection time of a pulse traveling across the narrowwidth of the human heel, for example, can produce unacceptablevariations in sound speed determination.

The present invention reduces variation in detection time by theextensive application of digital control techniques across the entiresignal chain. The received signal is digitized to be received by amicroprocessor allowing flexible numerical analysis of pulse arrivaltime tailored to the particular device, frequency range, and region ofinvestigation. Conversion of the received pulse into digital words forreceipt by the microprocessor, together with initialization of thetransmitted pulse by the microprocessor allows computer-stable timing ofthe transmission of the pulse. The microprocessor adjusts thetransmitted pulse strength and the gain of the receiver amplifier todynamically optimize signal strength. Digitization of the received pulsealso allows a single captured pulse to be used for both the purpose ofmeasuring velocity or time of flight of the ultrasonic signal and inanalyzing its attenuation through mathematical techniques such as thefast Fourier transform.

In combination these techniques rendered possible clinically accuratespeed of sound measurements of bone in vivo.

More specifically, the present invention provides an ultrasonicdensitometer for measurement of the human os calcis in vivo including anultrasonic signal generator producing a broad band electrical pulse ofultrasonic frequencies and a first ultrasonic transducer connected tothe ultrasonic signal generator for producing a corresponding acousticsignal directed along a transmission axis.

A second ultrasonic transducer receives the acoustic signals directedalong the transmission axis and relays them to an analog to digitalconverter converting the electrical signals to digital representations.A microprocessor communicating with the ultrasonic signal generator andthe analog to digital converter executes a stored program to initiatethe transmission of the acoustic pulse and to numerically analyze thedigital representation of the received acoustic signal as distorted bythe imposition of the human heel between the first and second ultrasonictransducers to measure a time of transmission of the ultrasonic pulsebetween the first and second transducers.

Thus it is one object of the invention to provide improved accuracy inthe measurement of time of transmission through the use of numericalanalysis which may accommodate for pulse distortion and noise.

It is another object of the invention to provide for the digital controlby a single microprocessor in both initiation of the transmission of theacoustic pulse and its receipt and analysis such as provides moreprecise measurement.

The ultrasonic signal generator may be a digitally controllableamplifier designed to create a pulsed output.

Thus it is another object of the invention to provide control by themicroprocessor of the output signal as well as the processing of thereceived signal.

The densitometer may include a digitally controllable automatic gaincontrol circuit connected between the second ultrasonic transducer andthe analog to digital converter to receive the electrical signal fromthis second ultrasonic transducer and receive a control signal from themicroprocessor. The microprocessor operating according to its storedprogram may control the amplification of the electronic signal from thesecond ultrasonic transducer prior to its receipt by the digital toanalog converter.

Thus it is another object of the invention to provide precise digitalgain control as may be necessary to optimize the sensitivity of thereceive transducer and the amplifier circuit to received acousticsignals.

It is yet another object of the invention to provide for a conversion ofthe received signal to a digital form without loss of resolution bycontrolling the gain to fully use the range of the A to D converter.

The microprocessor may further numerically analyze the digitalrepresentation of the received acoustic signal as distorted by the humanheel to measure a change in shape of the waveform.

Thus it is another object of the invention to provide an extremely moreaccurate machine that provides both a measurement of transit speed ofthe ultrasonic wave and its attenuation such as may provide alternativeviews of the bone integrity or which may be combined to provide a morerobust measurement of bone integrity. It has been determined that thesetwo measurements supplement each other.

The measurement of the time of flight of the pulse and the measurementof pulse shape may be in comparison to previously measured a standard.

Thus it is another object of the invention to simplify the comparison ofmeasurements to a standard by use of a microprocessor which may storeearlier and later measurements.

The densitometer may include a digital display communicating with themicroprocessor and the microprocessor may execute a stored program toprovide data to that display indicating the physical property of the oscalcis of the heel.

Thus it is another object of the invention to provide a densitometerhaving suitable accuracy for a digital display that provides a simple,quantitative and unambiguous measurement of bone integrity that may notbe obtained from visual display of waveforms or frequency measurementsin a sing around system.

The foregoing and other objects and advantages of the invention willappear from the following description. In this description, reference ismade to the accompanying drawings which form a part hereof and in whichthere is shown by way of illustration, a preferred embodiment of theinvention. Such embodiment does not necessarily represent the full scopeof the invention, however, and reference must be made therefore to theclaims for interpreting the scope of the invention.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is a perspective view of the ultrasound densitometer deviceconstructed in accordance with the present invention;

FIG. 2 is a perspective view of an acoustic coupler, two of which areshown in FIG. 1;

FIG. 3 is a front view of a transducer face from which acoustic signalsare transmitted or by which acoustic signals are received, the face ofthe other transducer being the mirror image thereof;

FIG. 4 is a schematic block diagram view of the circuitry of theultrasound densitometer device constructed in accordance with thepresent invention;

FIG. 5 illustrates the method of sampling a received waveform used bythe circuit of FIG. 4;

FIG. 6 is a schematic block diagram view of the circuitry of analternative embodiment of an ultrasound densitometer constructed inaccordance with the present invention;

FIG. 7 is a sample of an actual ultrasonic pulse and response from anultrasonic densitometer according to the present invention;

FIG. 8 is a sample plot of relative ultrasound pulse intensity overfrequency range;

FIG. 9 is a graph in frequency domain illustrating the shift inattenuation versus frequency characteristic of a measured object ascompared to a reference;

FIG. 10 is a perspective view of an alternative embodiment of thepresent invention showing a basin for receiving a patient's foot andhaving integral opposed ultrasonic transducers.

DETAILED DESCRIPTION OF THE INVENTION

Referring more particularly to the drawings, wherein like numbers referto like parts, FIG. 1 shows a portable ultrasound densitometer 10 formeasuring the physical properties and integrity of a member, such as abone, in vivo. The densitometer 10 as shown in FIG. 1 includes a handle11 with actuator button 12. Extending linearly from the handle 11 is aconnection rod 13. The densitometer 10 also includes a fixed arm 15 andan adjustable arm 16. The fixed arm 15 preferably is formed continuouslywith the connection rod 13, and therefore is connected to an end 17 ofthe connection rod 13. The adjustable arm 16 is slidably mounted on theconnection rod 13 between the handle 11 and a digital display 18 mountedon the rod 13. The knob 19 may be turned so as to be locked or unlockedto allow the adjustable arm 16 to be slid along the connection rod 13 sothat the distance between the arms 15 and 16 may be adjusted.

Connected at the end of the fixed arm 15 is a first (left) transducer 21and at the end of the adjustable arm 16 is a second (right) transducer21. As shown in FIGS. 1 and 2, each of the transducers 21 has mounted onit a respective compliant acoustic coupler 23 to acoustically couple thetransducer to the object being tested. The acoustic coupler 23 includesa plastic ring 24 and attached pad 26 formed of urethane or othercompliant material. FIG. 3 shows a face 28 of the first (left)transducer 21 which is normally hidden behind the compliant pad 26 ofthe acoustic coupler 23. The transducer face 28 normally abuts againstthe inner surface 29 of the pad 26 shown in FIG. 2. The transducer face28 shown in FIG. 3 includes an array of twelve transducer elementslabeled a-l. The second (right) transducer 21 includes a face 28 whichis the mirror image of that shown in FIG. 3.

FIG. 4 generally shows in schematic fashion the electronic circuitry 31of the densitometer 10, which is physically contained in the housing ofthe digital display 18.

An object 32 is placed between the two transducers 21 so that acousticsignals may be transmitted through the object. This object 32 representsa member, such as a bone, or some material with known acousticproperties such as distilled water or a neoprene reference block. Asshown in the embodiment illustrated in FIG. 4, the leftmost transducer21 is a transmit transducer and the rightmost transducer 21 a receivetransducer. In fact though, either or both of the transducers 21 may bea transmit and/or receive transducer. The transmit and receivetransducers 21 of the circuit of FIG. 4 are connected by element selectsignals 36 and 37 to a microprocessor 38. The microprocessor 38 isprogrammed to determine which one of the respective pairs of transducerelements a through l are to be transmitting and receiving at any onetime. This selection is accomplished by the element select signal lines36 and 37, which may be either multiple signal lines or a serial dataline to transmit the needed selection data to the transducers 21. Themicroprocessor 38 is also connected by a data and address bus 40 to thedigital display 18, a digital signal processor 41, a sampling analog todigital converter 42, and a set of external timers 43. Themicroprocessor 38 has “on board” electrically programmable non-volatilerandom access memory (NVRAM) and, perhaps as well, conventional RAMmemory, and controls the operations of the densitometer 10. The digitalsignal processor 41 has “on board” read-only memory (ROM) and performsmany of the mathematical functions carried out by the densitometer 10under the control of the microprocessor 38. The digital signal processor41 specifically includes the capability to perform discrete Fouriertransforms, as is commercially available in integrated circuit formpresently, so as to be able to convert received waveform signals fromthe time domain to the frequency domain. The microprocessor 38 anddigital signal processor 41 are interconnected also by the controlsignals 45 and 46 so that the microprocessor 38 can maintain controlover the operations of the digital signal processor 41 and receivestatus information back. Together the microprocessor 38 and the digitalsignal processor 41 control the electrical circuit 31 so that thedensitometer 10 can carry out its operations, which will be discussedbelow. An auditory feedback mechanism 48, such as an audio speaker, canbe connected to the microprocessor 38 through an output signal 49.

The external timer 43 provides a series of clock signals 51 and 52 tothe A/D converter 42 to provide time information to the A/D converter 42so that it will sample at timed intervals electrical signals which itreceives ultimately from the transmit transducer, in accordance with theprogram in the microprocessor 38 and the digital signal processor 41.The external timer 43 also creates a clock signal 53 connected to anexcitation amplifier 55 with digitally controllable gain. Timed pulsesare generated by the timer 43 and sent through the signal line 53 to theamplifier 55 to be amplified and directed to the transmit transducer 21through the signal line 56. The transmit transducer 21 converts theamplified pulse into an acoustic signal which is transmitted through theobject or material 32 to be received by the receive transducer 21 whichconverts the acoustic signal back to an electrical signal. Theelectrical signal is directed through output signal 57 to a receiveramplifier 59 which amplifies the electrical signal.

The excitation amplifier circuit 55 is preferably a digitallycontrollable circuit designed to create a pulsed output. Theamplification of the pulse can be digitally controlled in steps from oneto ninety-nine. In this way, the pulse can be repetitively increased inamplitude under digital control until a received pulse of appropriateamplitude is received at the receiver/amplifier circuit 59, where thegain is also digitally adjustable.

Connected to the receiver amplifier circuit 59 and integral therewith isa digitally controllable automatic gain control circuit which optimizesthe sensitivity of the receive transducer 21 and the amplifier circuit59 to received acoustic signals. The microprocessor 38 is connected tothe amplifier circuit and automatic gain control 59 through signal line60 to regulate the amplification of the amplifier circuit and gaincontrol 59. The amplified electric signals are directed through lead 61to the A/D converter 42 which samples those signals at timed intervals.The A/D converter 42 therefore in effect samples the received acousticsignals. As a series of substantially identical acoustic signals arereceived by the receive transducer 21, the A/D converter 42progressively samples an incremental portion of each successive signalwaveform. The microprocessor 38 is programmed so that those portions arecombined to form a digital composite waveform which is nearly identicalto a single waveform. This digitized waveform may be displayed on thedigital display 18, or processed for numerical analysis by the digitalsignal processor 41.

The densitometer constructed in accordance with FIGS. 1-4 can beoperated in one or more of several distinct methods to measure thephysical properties of the member, such as integrity or density. Thedifferent methods, as described in further detail below, depend both onthe software programming the operation of the microprocessor 34 as wellas the instructions given to the clinician as to how to use thedensitometer. The different methods of use may all be programmed into asingle unit, in which case a user-selectable switch may be provided toselect the mode of operation, or a given densitometer could beconstructed to be dedicated to a single mode of use. In any event, forthe method of use of the densitometer to measure the physical propertiesof a member to be fully understood, it is first necessary to understandthe internal operation of the densitometer itself.

In any of its methods of use, the densitometer is intended to be placedat some point in the process on the member whose properties are beingmeasured. This is done by placing the transducers 21 on opposite sidesof the member. To accomplish this, the knob 19 is loosened to allow theadjustable arm 16 to be moved so that the transducers 21 can be placedon opposite sides of the member, such as the heel of a human patient.The outside surfaces of the pads 26 can be placed against the heel ofthe subject with an ultrasound gel 35 or other coupling material placedbetween the pads 26 and subject 32 to allow for improved transmission ofthe acoustic signals between the member 32 and transducers 21. Once thetransducers 21 are properly placed on the member, the knob 19 may betightened to hold the adjustable arm 16 in place, with the transducers21 in spaced relation to each other with the member 32 therebetween. Theactuator button 12 may then be pressed so that acoustic signals will betransmitted through the member 32 to be received by the receivetransducer 21. The electronic circuit of FIG. 4 receives the electricalsignals from the receive transducer 21, and samples and processes thesesignals to obtain information on the physical properties and integrityof the member 32 in vivo. The microprocessor 38 is programmed toindicate on the digital display 18 when this information gatheringprocess is complete. Alternatively, the information may be displayed onthe digital display 18 when the information gathering process iscompleted. For example, the transit time of the acoustic signals throughthe member 32 could simply be displayed on the digital display 18.

Considering in detail the operation of the circuitry of FIG. 4, thegeneral concept is that the circuitry is designed to create anultrasonic pulse which travels from transmit transducer 21 through thesubject 32 and is then received by the receive transducer 21. Thecircuitry is designed to both determine the transit time of the pulsethrough the member 32, to ascertain the attenuation of the pulse throughthe member 32, and to be able to reconstruct a digital representation ofthe waveform of the pulse after it has passed through the member 32, sothat it may be analyzed to determine the attenuation at selectedfrequencies. To accomplish all of these objectives, the circuitry ofFIG. 4 operates under the control of the microprocessor 38. Themicroprocessor 38 selectively selects, through the element select signallines 36, a corresponding pair or a group of the elements a through l onthe face of each of the transducers 21. The corresponding elements oneach transducer are selected simultaneously while the remaining elementson the face of each transducer are inactive. With a given element, sayfor example element a selected, the microprocessor then causes theexternal timer 43 to emit a pulse on signal line 53 to the excitationamplifier circuit 55. The output of the excitation amplifier 55 travelsalong signal line 56 to element a of the transmit transducer 21, whichthereupon emits the ultrasonic pulse. The corresponding element a on thereceive transducer 21 receives the pulse and presents its output on thesignal line 57 to the amplifier circuit 59. What is desired as an outputof the A/D converter 42 is a digital representation of the analogwaveform which is the output of the single transducer element which hasbeen selected.

Unfortunately, “real time” sampling A/D converters which can operaterapidly enough to sample a waveform at ultrasonic frequencies arerelatively expensive. Therefore it is preferred that the A/D converter42 be an “equivalent time” sampling A/D converter. By “equivalent time”sampling, it is meant that the A/D converter 42 samples the output ofthe transducer during a narrow time period after any given ultrasonicpulse. The general concept is illustrated in FIG. 5. The typicalwaveform of a single pulse received by the receive transducer 21 andimposed on the signal line 57 is indicated by a function “f”. The samepulse is repetitively received as an excitation pulse and isrepetitively launched. The received pulse is sampled at a sequence oftime periods labeled t₀-t₁₀. In other words, rather than trying to do areal-time analog to digital conversion of the signal f, the signal issampled during individual fixed time periods t₀-t₁₀ after the transmitpulse is imposed, the analog value during each time period is convertedto a digital function, and that data is stored. Thus the total analogwaveform response can be recreated from the individual digital valuescreated during each time period t, with the overall fidelity of therecreation of the waveform dependent on the number of time periods twhich are sampled. The sampling is not accomplished during a single realtime pulse from the receive transducer 21. Instead, a series of pulsesare emitted from the transmit transducer 21. The external timer isconstructed to provide signals to the sampling A/D converter 42 alongsignal lines 51 and 52 such that the analog value sampled at time periodt₀ when the first pulse is applied to a given transducer element, thenat time t₁ during the second pulse, time t₂ during the third pulse, etc.until all the time periods are sampled. Only after the complete waveformhas been sampled for each element is the next element, i.e. element b,selected. The output from the A/D converter 42 is provided both to themicroprocessor 38 and to the signal processor 41. Thus the digitaloutput values representing the complex waveform f of FIG. 5 can beprocessed by the signal processor 41 after they are compiled for eachtransducer element. The waveform can then be analyzed for time delay orattenuation for any given frequency component with respect to thecharacteristic of the transmitted ultrasonic pulse. The process is thenrepeated for the other elements until all elements have been utilized totransmit a series of pulses sufficient to create digital datarepresenting the waveform which was received at the receive transducerarray 21. It is this data which may then be utilized in a variety ofmethods for determining the physical properties of the member. Dependingon the manner in which the densitometer is being utilized and the databeing sought, the appropriate output can be provided from either themicroprocessor 38 or the signal processor 41 through the digital display18.

Because the ultrasonic pulsing and sampling can be performed so rapidly,at least in human terms, the process of creating a sampled ultrasonicreceived pulse can optionally be repeated several times to reduce noiseby signal averaging. If this option is to be implemented, the process ofrepetitively launching ultrasonic pulses and sampling the receivedwaveform as illustrated in FIG. 5 is repeated one or more times for eachelement in the array before proceeding to the next element. Then thesampled waveforms thus produced can be digitally averaged to produce acomposite waveform that will have a lesser random noise component thanany single sampled waveform. The number of repetitions necessary tosufficiently reduce noise can be determined by testing in a fashionknown to one skilled in the art.

Having thus reviewed the internal operation of the densitometer of FIGS.1-4, it is now possible to understand the methods of use of thedensitometer to measure the physical properties of the member. The firstmethod of use involves measuring transit time of an ultrasonic pulsethrough a subject and comparing that time to the time an ultrasonicpulse requires to travel an equal distance in a substance of knownacoustic properties such as water. To use the densitometer in thisprocedure, the adjustable arm 16 is adjusted until the member of thesubject, such as the heel, is clamped between the transducers 21. Thenthe knob 19 is tightened to fix the adjustable arm in place. Theactuator button 12 is then pressed to initiate a pulse and measurement.Next the densitometer is removed from the subject while keeping the knob19 tight so that the distance between the transducers 21 remains thesame. The device 10 is then placed about or immersed in a standardmaterial 32 with known acoustic properties, such as by immersion in abath of distilled water. The actuator button 12 is pressed again so thatacoustic signals are transmitted from the transmit transducer 21 throughthe material 32 to the receive transducer 21. While it is advantageousto utilize the whole array of elements a through l for the measurementof the member, it may only be necessary to use a single pair of elementsfor the measurement through the standard assuming only that the standardis homogeneous, unlike the member. The signal profiles received by thetwo measurements are then analyzed by the microprocessor 38 and thesignal processor 41. This analysis can be directed both to thecomparative time of transit of the pulse through the subject as comparedto the standard and to the characteristics of the waveform in frequencyresponse and attenuation through the subject as compared to thestandard.

Thus in this method the densitometer may determine the physicalproperties and integrity of the member 32 by both or either of two formsof analysis. The densitometer may compare the transit time of theacoustic signals through the member with the transmit time of theacoustic signals through the material of known acoustic properties,and/or the device 10 may compare the attenuation as a function offrequency of the broadband acoustic signals through the member 32 withthe attenuation of corresponding specific frequency components of theacoustic signals through the material of known acoustic properties. The“attenuation” of an acoustic signal through a substance is thediminution of the ultrasonic waveform from the propagation througheither the subject or the standard. The theory and experiments usingboth of these methods are presented and discussed in Rossman, P.J.,Measurements of Ultrasonic Velocity and Attenuation In The Human OsCalcis and Their Relationships to Photon Absorptiometry Bone MineralMeasurements (1987) (a thesis submitted in partial fulfillment of therequirements for the degree of Master of Science at the University ofWisconsin-Madison). Tests have indicated that there exists a linearrelationship between ultrasonic attenuation (measured in decibels) (dB))at specific frequencies, and those frequencies. The slope (dB/MHz) ofthe linear relationship, referred to as the broadband ultrasonicattenuation, is dependent upon the physical properties and integrity ofthe substance being tested. With a bone, the slope of the linearrelationship would be dependent upon the bone mineral density. Thusbroadband ultrasonic attenuation through a bone is a parameter directlyrelated to the quality of the cancellous bone matrix.

The microprocessor 38 may therefore be programmed so that the devicedetermines the physical properties and integrity of the member bycomparing either relative transit times and/or relative broadbandultrasonic attenuation through the member and a material of knownacoustic properties. When comparing the transit times, themicroprocessor 38 may be programmed most simply so that the electronics,having received the acoustic signals after they have been transmittedthrough the member, determines the “member” transit time of thoseacoustic signals through the member, and after the acoustic signals havebeen transmitted through the material of known acoustic properties,determines the “material” transit time of the acoustic signals throughthe material. These time periods may be measured most simply by countingthe number of clock pulses of known frequency emitted by the timer 43between the time of launching the pulse and the sensing of the receivedpulse at the A/D converter 42. The microprocessor 38 then makes amathematical “time” comparison of the member transit time to thematerial transit time and then relates that mathematical time comparisonto the physical properties and integrity of the member. The mathematicaltime comparison may be made by either determining a difference betweenthe member transit time and the material transit time, or by determininga ratio between the member transit time and the material transit time.

As a second method of using the densitometer, it may also determine thephysical properties and integrity of the member 32 by determining andcomparing the attenuation of the broadband frequency components of theacoustic signals through the member without reference to a materialhaving known acoustic properties. Using this method, the comparison ofvelocity to a standard is not necessary and absolute transit time of thepulse need not be calculated since it is attenuation that is measured.In such a mode, it is preferable that the transmit transducer 21transmits an acoustic signal which has a broad range of frequencycomponents, such as a simple ultrasonic pulse. In any case, the acousticsignal should have at least one specific frequency component.

In this attenuation comparison mode, the microprocessor 38 is programmedso that after the receive transducer 21 receives the acoustic signalstransmitted through the bone member 32, it determines the absoluteattenuation through the member 32 of the frequency component spectrum ofthe acoustic signals. It is to facilitate the measurement of attenuationthat the excitation amplifier circuit 55 and the receiver amplifier 59have amplification levels which may be digitally controlled. Bysuccessively varying the gain of the amplifiers 55 and 59 on successivepulses, the circuit of FIG. 4 can determine what level of gain isnecessary to place the peak of the received waveform at a proper voltagelevel. This gain is, of course, a function of the level of attenuationof the acoustic pulse during transit through the member 32. After thereceive transducer 21 receives acoustic signals, microprocessor 38 inconjunction with the signal processor 41 determines the absoluteattenuation of individual specific frequency components of the receivedacoustic signal transmitted through the material. The digital signalprocessor 41 then makes mathematical “attenuation” comparisons of thecorresponding individual specific frequency components through themember. A set of mathematical attenuation comparisons betweencorresponding frequency components may be thereby obtained, onecomparison for each frequency component compared. The manner in whichthe attenuation functions with respect to frequency can thus be derived.The microprocessor 38 and digital signal processor 41 then relate thatfunction to the physical properties and integrity of the member.

Shown in FIG. 7 is a sample broadband ultrasonic pulse and a typicalreceived waveform. To achieve an ultrasonic signal that is very broad inthe frequency domain, i.e., a broadband transmitted signal, anelectronic pulse such as indicated at 70 is applied to the selectedultrasonic transducer in the transmit array 21 which then resonates witha broadband ultrasonic emission. The received signal, such as indicatedat 72 in FIG. 7 in a time domain signal plot, is then processed bydiscrete Fourier transform analysis so that it is converted to thefrequency domain. Shown in FIG. 8 is a pair of plots of sample receivedsignals, in frequency domain plots, showing the shift in received signalintensity as a function of frequency between a reference object and aplug of neoprene placed in the instrument. FIG. 9 illustrates a similarcomparison, with FIG. 8 using relative attenuation in the verticaldimension and FIG. 9 using power of the received signal using a similarreference material. Both representations illustrate the difference inrelative intensities as a function of frequency illustrating bowbroadband ultrasonic attenuation varies from object to object. Theactual value calculated, broadband ultrasonic attenuation, is calculatedby first comparing the received signal against the reference signal,then performing the discrete Fourier transform to convert to frequencydomain, then performing a linear regression of the difference inattenuation slope to derive broadband ultrasonic attenuation.

The mathematics of the discrete Fourier transform are such that anotherparameter related to bone member density may be calculated in additionto, or in substitution for, broadband attenuation (sometimes referred toas “attenuation” or “BUA” below). When the discrete Fourier transform isperformed on the time-domain signal, the solution for each pointincludes a real member component and an imaginary member component. Thevalues graphed in FIGS. 8 and 9 are the amplitude of the received pulseas determined from this discrete Fourier transform by taking the squareroot of the sum of the squares of the real component and the imaginarycomponent. The phase angle of the change in phase of the ultrasonicpulse as it passed through the member can be calculated by taking thearctangent of the ratio of the imaginary to the real components. Thisphase angle value is also calculated to bone member density.

The microprocessor 38 may also be programmed so that the densitometersimultaneously performs both functions, i.e. determines both transittime and absolute attenuation of the transmitted acoustic signals, firstthrough the member and then through the material with known acousticproperties. The densitometer may then both derive the broadbandultrasonic attenuation function and make a mathematical time comparisonof the member transit time to the material transit time. Themicroprocessor 38 and digital signal processor 41 then relate both thetime comparison along with the attenuation function to the physicalproperties and integrity, or density of the member 32.

In yet another possible mode of operation, the microprocessor 38 may beprogrammed so that the densitometer 10 operates in a mode whereby theneed for calculating either the relative transit time or the attenuationof the acoustic signals through a material of known acoustic propertiesis eliminated. In order to operate in such a mode, the microprocessor 38would include a database of normal absolute transit times which arebased upon such factors as the age, height, weight, race or the sex ofthe individual being tested as well as the distance between thetransducers or the thickness or size of the member. This database ofnormal transit times can be stored in the non-volatile memory or couldbe stored in other media. When testing an individual in this mode, therelevant factors for the individual are placed into the microprocessor38 to select the pertinent normal transit time based on those factors.The transducers 21 are placed on the bone member being tested asdescribed above. When the actuator button 12 is pressed, the acousticsignals are-transmitted through the member 32. The receive transducer 21receives those signals after they have been transmitted through themember, and the electronics 31 then determine the “member” transit timeof the acoustic signals through the member. The microprocessor 38 anddigital signal processor 41 then make a mathematical comparison of themeasured member transit time to the selected database normal transittime, and relate the mathematical time comparison to the physicalproperties and integrity, or density of the member, which is displayed.

As an alternative output of the densitometer of the present invention,the digital display 18 could also include a display corresponding to thepattern of the array of elements on the face of the transducer 21 asseen in FIG. 3. This display could then display, for each element athrough l, a gray scale image proportional to the parameter, i.e.transit time or attenuation, being measured. This image may provide avisual indication to an experienced clinician as to the physicalproperties of the member present in the patient.

Shown in FIG. 6 is a circuit schematic for an alternative embodiment ofan ultrasonic densitometer constructed in accordance with the presentinvention. In the circuit of FIG. 6, parts having similar structure andfunction to their corresponding parts in FIG. 4 are indicated withsimilar reference numerals.

The embodiment of FIG. 6 is intended to function with only a singletransducer array 21 which functions both as the transmit and the receivetransducer array. An optional reflecting surface 64 may be placed on theopposite side of the member 32 from the transducer array 21. A digitallycontrolled multiple pole switch 66, preferably an electronic switchrather than a mechanical one, connects the input to and output from theelements of the transducer array 21 selectively either to the excitationamplifier 55 or to the controllable gain receiver/amplifier circuit 59.The switch 66 is connected by a switch control line 68 to an output ofthe microprocessor 38.

In the operation of the circuit of FIG. 6, it functions in most respectslike the circuit of FIG. 4, so only the differences need be discussed.During the launching of an ultrasonic pulse, the microprocessor 38causes a signal to appear on the switch control line 68 to cause theswitch 66 to connect the output of the excitation amplifier 55 to theselected element in the transducer array 21. Following completion of thelaunching of the pulse, the microprocessor 38 changes the signal on theswitch control line 68 to operate the switch 66 to connect the selectedelement or elements as an input to the amplifier 59. Meanwhile, thepulse propagates through the member 32. As the pulse transits throughthe member, reflective pulses will be generated as the pulse crossesinterfaces of differing materials in the member and, in particular, asthe pulse exits the member into the air at the opposite side of themember. If the transition from the member to air does not produce asufficient reflective pulse, the reflecting surface 64 can be placedagainst the opposite side of the member to provide an enhanced reflectedpulse.

The embodiment of FIG. 6 can thus be used to analyze the physicalproperties and integrity of a member using only one transducer 21. Allof the methods described above for such measurements may be used equallyeffectively with this version of the device. The transit time of thepulse through the member can be measured simply by measuring the timeperiod until receipt of the reflected pulse, and then simply dividing bytwo. This time period can be compared to the transit time, over asimilar distance, through a standard medium such as water. The timeperiod for receipt of the reflected pulse could also be simply comparedto standard values for age, sex, etc. Attenuation measurements to detectdifferential frequency measurement can be directly made on the reflectedpulse. If no reflecting surface 64 is used, and it is desired todetermine absolute transit time, the thickness of the member or samplecan be measured.

The use of the multi-element ultrasonic transducer array for thetransducers 21, as illustrated in FIG. 3, enables another advantageousfeature of the instrument of FIGS. 1-9. In using prior artdensitometers, it was often necessary to precisely position theinstrument relative to the body member of the patient being measured tohave useful results. The difficulty arises because of heterogeneities inthe bone mass and structure of actual body members. A measurement takenat one location of density may be significantly different from ameasurement taken close by. Therefore prior art instruments fixed thebody member precisely so that the measurement could be taken at theprecise location each time.

The use of the ultrasonic transducer array obviates the need for thisprecise positioning. Using the instrument of FIGS. 1-9, the instrumentperforms a pulse and response, performs the discrete Fourier transform,and generates a value for broadband ultrasonic attenuation for each pairof transducer elements a through l. Then the microprocessor 38 analyzesthe resulting array of bone ultrasonic density measurements toreproducibly identify the same region of interest each time. In otherwords, since the physical array of transducers is large enough toreliably cover at least the one common region of interest each time, themeasurement is localized at the same locus each time by electricallyselecting the proper location for the measurement from among thelocations measured by the array. The instrument of FIGS. 1-9 isconveniently used by measuring the density of the os calcis as measuredthrough the heel of a human patient. When used in this location, it hasbeen found that a region of interest in the os calcis can be locatedreliably and repeatedly based on the comparisons of broadband ultrasonicattenuation at the points in the array. The region of interest in the oscalcis is identified as a local or relative minimum in broadbandultrasonic attenuation and/or velocity closely adjacent the region ofhighest attenuation values in the body member. Thus repetitivemeasurements of the broadband ultrasonic attenuation value at this sameregion of interest can be reproducibly taken even though thedensitometer instrument 10 is only generally positioned at the samelocation for each successive measurement.

This technique of using a multiple element array to avoid positioncriticality is applicable to other techniques other than thedetermination of broadband ultrasonic attenuation as described here. Theconcept of using an array and comparing the array of results todetermine measurement locus would be equally applicable to measurementstaken of member-density based on speed of sound transit time, othermeasurements of attenuation or on the calculation of phase anglediscussed above. The use of such a multiple-element array, withautomated selection of one element in the region of interest, can alsobe applied to other measurement techniques useful for generatingparameters related to bone member density, such as measuring speedchanges in the transmitted pulse such as suggested in U.S. Pat. No.4,361,154 to Pratt, or measuring the frequency of a “sing-around”self-triggering pulse as suggested in U.S. Pat. No. 3,847,141 to Hoop.The concept which permits the position independence feature is that ofan array of measurements generating an array of data points from which aregion of interest is selected by a reproducible criterion or severalcriteria. The number of elements in the array also clearly can be variedwith a larger number of elements resulting in a greater accuracy inidentifying the same region of interest.

In this way, the ultrasound densitometer of the present inventionprovides a device capable of rapid and efficient determination of thephysical properties of a member in vivo without the use of radiation.Because the densitometer is constructed to operate under the control ofthe microprocessor 38, it can be programmed to operate in one of severalmodes, as discussed above. This allows both for flexibility to clinicalgoals as well as efficient use of the device.

Shown in FIG. 10 is another variation on an ultrasonic densitometerconstructed in accordance with the present invention. In thedensitometer 100 of FIG. 10, there are two ultrasonic transducer arrays121, which are generally similar to the ultrasonic transducer arrays 21of the embodiment of FIG. 1, except that the transducer arrays 21 arefixed in position rather than movable.

The densitometer 100 includes a relatively large mounting case 101,which in this embodiment is rectangular in the top of which is formed adepression or basin 103. The basin as in 103 is elongated and has agenerally triangular cross-sectional shape sized so as to receive ahuman foot therein. The transducer arrays 121 are positioned in the case101 so that they extend into the base in 103 to be on the opposite sidesof the heel of a foot placed in the basin 103. The general arrangementof the electrical components of FIG. 4 is unchanged in the ultrasonicdensitometer 100 of FIG. 10, except that because the transducer arrays12 are fixed, they do not contact the opposite sides of the heel of thepatient but instead are located spaced away from it.

In the operation of the ultrasonic densitometer 100 of FIG. 10, thebasin 103 is filled with water. The patient places his or her foot inthe basin 103 with their sole resting on the bottom and their heelresting against the back of the basin 103. The intent is to position thetransducer arrays 121 on either side of the os calcis. For the averageadult population, it has been demonstrated that placing the transducersapproximately 4 cm up from the sole and 3.5 cm forwardly from therearward edge of the heel places the transducers in the desired regionand focused on the os calcis.

The principle of operation of the ultrasonic densitometer of FIG. 10 canbe understood as similar to that of FIG. 1, except that the order ofpulsing and measurement can be varied. In the apparatus of FIG. 1, themeasurement pulse through the member was generally performed before thereference pulse through the homogeneous standard, i.e. water. In thedensitometer 100 of FIG. 10, since the distance between the transducersis fixed, the reference pulse through the homogeneous standard material,which is simply the water in the basin 103, may be conducted eitherbefore or after measurement pulse through a live member is performed. Infact, if the temperature of the water in basin 103 is held steady by atemperature control mechanism, the standard transit time measurement canbe simply made once for the instrument and thereafter only measurementpulses need be transmitted. The transit times of the measurement pulsesthen may be correlated to the standard reference transit time throughthe water to give an indication of the integrity of the member justmeasured. Through empirical experimentation with the densitometer ofFIG. 10, it has been determined that it is also possible if desired todispense with the reference pulse entirely. This may be done bydetermining a standard transit time value or, since the distance betweenthe transducers is fixed, a standard transit velocity. By empiricaltesting, it has been determined that by proper selection of such astandard value, and by holding the water in the basin within atemperature range, no reference pulse need be launched or measured.

Using this variation, a mathematical relation of the measured transittime, or transit velocity, must be made to the standard. Since in theinterests of accuracy, it is preferred to use both changes in transittime (velocity) and changes in attenuation to evaluate a member in vivo,the following formula has been developed to provide a numerical valueindicative of the integrity and mineral density of a bone:

Bone integrity value=(SOS−T)² ×(BUA/1000)

In this formula, “SOS” indicates the speed of sound or velocity of themeasurement ultrasonic pulse through the member and is expressed inmeters per second. The speed of sound (SOS) value is calculated from themeasured transit time measured. For an adult human heel, it has beenfound that assuming a standard human heel width of 40 mm at the point ofmeasurement results in such sufficient and reproducible accuracy thatactual measurement of the actual individual heel is not needed.

In the above formula, “T” represents a standard minimum value. Twoalternative values are possible. One alternative is to set T to thespeed of sound value for water, i.e., the reference pulse velocity. Thisvalue is about 1500 n/sec for water at 28° C. The principal drawback tothis approach is that it has been found surprisingly that some peopleactually have a density value in their heel that is below that of water.For such persons, using the standard water velocity would make the boneintegrity value a negative number. Therefore, another alternative is touse the lowest measured human value as T which in the experience of theinvestigators here to date is 1475 n/sec.

Lastly in the above formula, BUA is broadband ultrasonic attenuation asdescribed in greater detail above. The division of 1000 merely scalesthe influence of the BUA measurement relative to the SOS measurementwhich has been determined to be a more effective predictor of bonedensity.

Measured values of SOS range between 1475 and 1650 m/sec. Measuredvalues of BUA range between 30 and 100 dB/MHz. Using a T=1475, theseranges yield values ranging from very small, i.e., 18 up to relativelylarge, i.e., around 3000. Thus the bone integrity values thus obtainedexhibit a wide range and are readily comprehensible. It has beendetermined again by clinical testing that persons with a bone integrityvalue of less than 200 have low spinal bone mineral density, that thosein the range of 200-400 have marginal spinal bone mineral density andthat those having bone integrity values of over 400 have acceptable andhigh levels of spinal bone mineral density.

To verify the accuracy of this approach in predicting spinal bonedensity, patients were tested using the apparatus of FIG. 10 and alsowith a dual photon absorptiometry densitometer of accepted standarddesign. The results of using the ultrasonic densitometer of FIG. 10 havedemonstrated that the speed of sound measurement made using this devicehad a correlation in excess of 0.95 with the measured values of spinalbone density indicating very good consistency with accepted techniques.However, an occasional patient was tested who exhibited an SOS value inthe normal range, but who exhibited a BUZ value indicating very poorbone integrity. Accordingly, the bone integrity value was developed toaccommodate such deviant results. The value is weighted toward the SOSsince that is the principally used reliable predictor value with asecondary factor including BUA to include such individuals. In fact, thepower of the SOS factor may also be increased to the third or fourthpower as opposed to merely the second power, to increase the importanceof the SOS term. Since this method utilizing ultrasonic measurement ofthe heel is quick and free from radiation, it offers a promisingalternative for evaluation of bone integrity.

The densitometer 100 may be used with or without an array of ultrasonictransducers in the transducers 121. In its simplest form, the mechanicalalignment of the heel in the device can be provided by the shape andsize of the basin 103. While the use of an array and region-of-interestscanning as described above is most helpful in ensuring a reproducibleaccurate measurement, mechanical placement may be acceptable forclinical utility in which case only single transducer elements arerequired.

It is specifically intended that the present invention not bespecifically limited to the embodiment and illustrations containedherein, but embrace all such modified forms thereof as come within thescope of the following claims.

What is claimed is:
 1. An ultrasonic instrument for measurement of thehuman os calcis in vivo comprising: signal generation circuitryproducing a transducer signal; a first ultrasonic transducer receivingthe transducer signal to produce an ultrasonic signal directed along atransmission axis; a second ultrasonic transducer positioned along thetransmission axis opposite the first ultrasonic transducer receiving theultrasonic signal to produce an electrical signal; an analog to digitalconverter receiving the electrical signal to produce a digitized signal;a microprocessor communicating with the signal generation circuitry andthe analog to digital converter, the microprocessor further executing astored program to: (i) monitor timing of transmission of the ultrasonicsignal after a human heel is placed between the first and secondtransducers; and (ii) numerically analyze the digitized signal, as byaffected by the imposition of a human heel between the first and secondultrasonic transducer, to determine the time of arrival of theultrasonic signal and compute a bone quality measurement.
 2. Theultrasonic instrument of claim 1 wherein the signal generation circuitryincludes a digitally controlled amplifier controlled by themicroprocessor to adjust the amplification of the ultrasonic signal. 3.The ultrasonic instrument of claim 1 including further a digitallycontrollable amplifier connected between the second ultrasonictransducer and the analog to digital converter to receive the electricalsignal from the second ultrasonic transducer and to receive a controlsignal from the microprocessor; and wherein the microprocessor operatesaccording to the stored program to control the amplification of theelectronic signal from the second ultrasonic transducer prior to itsreceipt by the analog to digital converter.
 4. The ultrasonic instrumentof claim 1 wherein the microprocessor further executes the storedprogram to: (i) monitor the initiation of the transmission of an otherultrasonic signal when a standard material of known acoustic propertiesis placed between the first and second ultrasonic transducers; and (ii)numerically analyze a second digitized signal, representing the otherultrasonic signal as affected by the imposition of the standardmaterial, to measure a time of transmission of the ultrasonic signalthrough the standard material between the first and second transducersto compare the measures of time through the human heel and the measuresof time through the standard material.
 5. The ultrasonic instrument ofclaim 1 wherein the microprocessor further executes the stored programto: (i) numerically analyze the digitized signal, as affected by theimposition of a human heel between the first and second ultrasonictransducer, to measure a change in shape of the digitized signal.
 6. Theultrasonic instrument of claim 5 wherein the numerical analysis tomeasure a change in shape of the digitized signal with respect to astandard signal.
 7. The ultrasonic instrument of claim 6 wherein thenumerical analysis to measure a change in shape of the waveform providesan indication of attenuation of specific frequency components throughthe human heel.
 8. The ultrasonic instrument of claim 1 wherein themicroprocessor executes the stored program to: (i) monitor timing oftransmission of an other ultrasonic signal; and (ii) numerically analyzea second digitized signal representing the other ultrasonic signal asaffected by the imposition of a human heel between the first and secondultrasonic transducer, to measure a change in shape of the seconddigitized signal.
 9. The ultrasonic instrument of claim 1 wherein themicroprocessor executes the stored program to: (i) monitor the timing ofthe transmission of a plurality of ultrasonic signals; and (ii) averagea plurality of digital representations of the received ultrasonicsignals, as affected by the imposition of a human heel between the firstand second ultrasonic transducers.
 10. The ultrasonic instrument ofclaim 1 further including a digital display communicating with themicroprocessor and wherein the microprocessor further executes thestored program to provide data to the display indicating a physicalproperty of the os calcis of the heel.
 11. A method for measurement ofthe human os calcis in vivo comprising the steps of: (a) generating atransducer signal; (b) communicating the transducer signal to a firstultrasonic transducer to produce an ultrasonic signal directed along atransmission axis through a human heel; (c) receiving at a secondultrasonic transducer, the ultrasonic signal after passage through thehuman heel; (d) receiving at an analog to digital converter anelectrical signal from the second ultrasonic transducer to produce adigitized signal; and (e) numerically analyzing the digitized signal tomeasure a time of transmission of the ultrasonic signal between thefirst and second transducers and compute a bone quality measurement. 12.The method of claim 11 including further the steps of monitoring andadjusting the amplification of the transducer signal based on thedigitized signal.
 13. The method of claim 11 including further the stepsof monitoring and adjusting an amplification of the electrical signalbased on the digitized signal.
 14. The method of claim 11 including thefurther steps of: monitoring the timing of the transmission of an otherultrasonic signal when a standard material of known acoustic propertiesis placed between the first and second ultrasonic transducers; andnumerically analyzing the second digitized signal representing the otherultrasonic signal, as affected by the imposition of the standardmaterial, to measure a time of transmission of the ultrasonic signalthrough the standard material between the first and second transducersto compare the measures of time through the human heel and the measuresof time through the standard material.
 15. The method of claim 11further including the step of numerically analyzing the digitized signalto measure a change in shape of the digitized signal.
 16. The method ofclaim 15 wherein the step of numerical analysis measures the change inshape of the digitized signal with respect to a standard signal.
 17. Themethod of claim 16 wherein the step of measuring the change in shape ofthe digitized signal provides an indication of attenuation of specificfrequency components through the human heel.
 18. The method of claim 11including the steps of: generating a second transducer signal;communicating the second transducer signal to the first ultrasonictransducer to produce a second ultrasonic signal directed along atransmission axis through a human heel; receiving the second ultrasonicsignal after passage through the human heel at the second ultrasonictransducer; receiving a second electrical signal from the secondultrasonic transducer with an analog to digital converter to produce asecond digitized signal; and numerically analyzing the second digitizedsignal of the second ultrasonic signal to measure a change in shape ofthe second digitized signal.
 19. The method of claim 11 includingfurther the steps of: repeating all but the final step of claim 11 andfurther including the step of: digitally averaging a plurality ofdigitized signals, as affected by the imposition of a human heel betweenthe first and second ultrasonic transducer, to produce a compositewaveform for the final step of numeric analyses.
 20. The method of claim11 further including the step of displaying a physical property of theos calcis of the heel based on the numerical analysis.
 21. An ultrasonicinstrument for measurement of the human os calcis in vivo comprising:signal generation circuitry producing a transducer signal controllablein amplitude by a control signal; a first ultrasonic transducerconnected to the signal generation circuitry to produce an ultrasonicsignal directed along a transmission axis; a second ultrasonictransducer positioned along the transmission axis opposite the firstultrasonic transducer receiving ultrasonic signal directed along thetransmission axis to produce an electrical signal; an analog to digitalconverter receiving an electrical signal to produce a digitized signal;and a microprocessor communicating with the signal generation circuitryand the analog to digital converter and executing a stored program to:(i) monitor timing of transmission of the ultrasonic signal after ahuman heel is placed between the first and second transducers; and (ii)monitor the received ultrasonic signal to determine the ultrasonicsignal's time of arrival at the second transducer after transmissionfrom the first transducer; and (iii) based on the received ultrasonicsignal, provide a control signal to the signal generation circuitryadjusting the amplitude of the electrical ultrasonic signal and computea bone quality measurement.
 22. The ultrasonic instrument of claim 21including further a digitally controllable amplifier connected betweenthe second ultrasonic transducer and the analog to digital converter toreceive the electrical signal from the second ultrasonic transducer andreceiving a second control signal from the microprocessor; and whereinthe microprocessor operates according to the stored program to further:based on the received ultrasonic signal, provide a second control signalto the digitally controllable amplifier to control the amplification ofthe electronic signal from the second ultrasonic transducer prior to itsreceipt by the analog to digital converter.
 23. The ultrasonicinstrument of claim 21 wherein the microprocessor further executes thestored program to: (i) monitor timing of the transmission of anotherultrasonic signal when a standard material of known acoustic propertiesis placed between the first and second ultrasonic transducers; and (ii)numerically analyze the second digitized signal representing the otherultrasonic signal as affected by the imposition of the standardmaterial, to measure a time of transmission of the ultrasonic signalthrough the standard material between the first and second transducersto compare the measures of time through the human heel and the measuresof time through the standard material.
 24. The ultrasonic instrument ofclaim 21 wherein the microprocessor further executes the stored programto: (i) numerically analyze the digitized signal, as affected by theimposition of a human heel between the first and second ultrasonictransducer, to measure a change in shape of the waveform.
 25. Theultrasonic instrument of claim 24 wherein the numerical analysis tomeasure a change in shape of the waveform measures the change in shapeof the waveform with respect to a standard waveform.
 26. The ultrasonicinstrument of claim 25 wherein the numerical analysis to measure achange in shape of the waveform provides an indication of attenuation ofspecific frequency components through the human heel.
 27. The ultrasonicinstrument of claim 21 wherein the microprocessor further executes thestored program to: (i) monitor timing of transmission of an otherultrasonic signal; and (ii) numerically analyze a second digitizedsignal representing the other ultrasonic signal as affected by theimposition of a human heel between the first and second ultrasonictransducer to measure a change in shape of the waveform.
 28. Theultrasonic instrument of claim 21 wherein the microprocessor executesthe stored program to: (i) monitor timing of transmission of a pluralityof transmissions of the ultrasonic signal; and (ii) average a pluralityof digitized signals, as affected by the imposition of a human heelbetween the first and second ultrasonic transducer, to produce acomposite waveform for numeric analyses.
 29. The ultrasonic instrumentof claim 21 further including a digital display communicating with themicroprocessor and wherein the microprocessor further executes thestored program to provide data to the display indicating a physicalproperty of the os calcis of the heel.
 30. A method for measurement ofthe human os calcis in vivo comprising the steps of: (a) generating abroadband transducer signal controllable in amplitude by a controlsignal; (b) communicating the transducer signal to a first ultrasonictransducer to produce an ultrasonic signal directed along a transmissionaxis through a human heel; (c) receiving the ultrasonic signal afterdistortion by the human heel at a second ultrasonic transducer toproduce an electrical signal; (d) receiving an electrical signal fromthe second ultrasonic transducer with an analog to digital converter toproduce a second digitized signal; and (e) based on the receivedultrasonic signals provide a control signal to the signal generationcircuitry adjusting the amplitude of the electrical ultrasonic signaland compute a bone quality measurement.
 31. The method of claim 30including further the step of providing a control to a gain controllableamplifier connected between the second ultrasonic transducer and theanalog to digital converter to receive the electrical signal from thesecond ultrasonic transducer, to control the amplification of theelectronic signal from the second ultrasonic transducer prior to itsreceipt by the analog to digital converter.
 32. The method of claim 30including the further steps of: monitoring the timing of thetransmission of an other ultrasonic signal when a standard material ofknown acoustic properties is placed between the first and secondultrasonic transducers; and numerically analyzing a second digitizedsignal representing the other ultrasonic signal as affected by theimposition of the standard material, to measure a time of transmissionof the ultrasonic signal through the standard material between the firstand second transducers to compare the measures of time through the humanheel and the measures of time through the standard material.
 33. Themethod of claim 30 further including the step of numerically analyzingthe digitized signal to measure a change in shape of the waveform. 34.The method of claim 33 wherein the step of numerical analysis measuresthe change in shape of the waveform with respect to a standard waveform.35. The method of claim 34 wherein the step of measuring the change inshape of the waveform provides an indication of attenuation of specificfrequency components through the human heel.
 36. The method of claim 30including the steps of: generating a second broadband transducer signalunder the control of a microprocessor; communicating the secondtransducer signal to the first ultrasonic transducer to produce a secondultrasonic signal directed along a transmission axis through a humanheel; receiving the second ultrasonic signal after distortion by thehuman heel at the second ultrasonic transducer; receiving a secondelectrical signal from the second ultrasonic transducer with an analogto digital converter to produce a second digitized signal; andnumerically analyzing the second digitized signal of the secondultrasonic signal to measure a change in shape of the waveform.
 37. Themethod of claim 30 including further the steps of: repeating steps(a)-(d) of claim 30 and wherein step (e) further includes the step of:digitally averaging a plurality of digitized signals, as affected by theimposition of a human heel between the first and second ultrasonictransducer, to produce a composite waveform for numeric analyses. 38.The method of claim 30 further including the step of displaying aphysical property of the os calcis of the heel based on the numericalanalysis.
 39. An ultrasonic instrument for measurement of the human oscalcis in vivo comprising: signal generation circuitry producing atransducer signal; a first ultrasonic transducer receiving thetransducer signal to produce an ultrasonic signal directed along atransmission axis; a second ultrasonic transducer positioned along thetransmission axis opposite the first ultrasonic transducer receiving anultrasonic signal directed along the transmission axis; a digitallycontrollable amplifier receiving the electrical signal from the secondultrasonic transducer and an amplitude control signal from themicroprocessor; an analog to digital converter receiving an electricalsignal from the digitally controllable amplifier to produce a digitizedsignal; and a microprocessor communicating with the signal generationcircuitry and the analog to digital converter and executing a storedprogram to: (i) monitor timing of transmission of the ultrasonic signalafter a human heel is placed between the first and second transducers;(ii) monitor the received ultrasonic signal to determine the ultrasonicsignal's time of arrival at the second transducers after transmissionfrom the first transducer; and (iii) based on the received ultrasonicsignals provide a control signal controlling the amplification of theelectronic signal from the second ultrasonic transducer prior to itsreceipt by the analog to digital converter and compute a bone qualitymeasurement.
 40. The ultrasonic instrument of claim 39 wherein thesignal generation circuitry includes a digitally controlled amplifiercontrolled by the microprocessor to adjust the ultrasonic signaltransmitted by the first ultrasonic transducer.
 41. The ultrasonicinstrument of claim 39 wherein the microprocessor further executes thestored program to: (i) monitor the initiation of the transmission of another ultrasonic signal when a standard material of known acousticproperties is placed between the first and second ultrasonictransducers; and (ii) numerically analyze a second digitized signalrepresenting the other ultrasonic signal as affected by the impositionof the standard material, to measure a time of transmission of theultrasonic signal through the standard material between the first andsecond transducers to compare the measures of time through the humanheel and the measures of time through the standard material.
 42. Theultrasonic instrument of claim 39 wherein the microprocessor furtherexecutes the stored program to: (i) numerically analyze the digitizedsignal, as affected by the imposition of a human heel between the firstand second ultrasonic transducers, to measure a change in shape of thewaveform.
 43. The ultrasonic instrument of claim 42 wherein thenumerical analysis to measure a change in shape of the waveform measuresthe change in shape of the waveform with respect to a standard waveform.44. The ultrasonic instrument of claim 43 wherein the numerical analysisto measure a change in shape of the waveform provides an indication ofattenuation of specific frequency components through the human heel. 45.The ultrasonic instrument of claim 44 wherein the microprocessor furtherexecutes the stored program to: (i) monitor timing of transmission of another ultrasonic signal; and (ii) numerically analyze the digitizedsignal representing the other ultrasonic signal, as affected by theimposition of a human heel between the first and second ultrasonictransducer, to measure a change in shape of the waveform.
 46. Theultrasonic instrument of claim 39 wherein the microprocessor executesthe stored program to: (i) monitor timing of transmission of a pluralityof transmissions of the ultrasonic signal; and (ii) average a pluralityof digitized signals, as affected by the imposition of a human heelbetween the first and second ultrasonic transducer, to produce acomposite waveform for numeric analyses.
 47. The ultrasonic instrumentof claim 39 further including a digital display communicating with themicroprocessor and wherein the microprocessor further executes thestored program to provide data to the display indicating a physicalproperty of the os calcis of the heel.
 48. A method for measurement ofthe human os calcis in vivo comprising the steps of: (a) generating atransducer signal; (b) communicating the transducer signal to a firstultrasonic transducer to produce an ultrasonic signal directed along atransmission axis through a human heel; (c) receiving the ultrasonicsignal after distortion by the human heel at a second ultrasonictransducer; (d) providing a gain controllable amplifier to receive theelectrical signal from the second ultrasonic transducer and to controlthe amplification of the electronic signal according to a controlsignal; (e) receiving an electrical signal from the gain controllableamplifier with an analog to digital converter to produce a digitizedsignal; and (f) based on the received ultrasonic signals provide acontrol signal to the gain controllable amplifier adjusting theamplitude of the step of providing a control signal from themicroprocessor to a gain controllable amplifier to control theamplification of the electronic signal from the second ultrasonictransducer prior to its receipt by the analog to digital converter andcompute a bone quality measurement.
 49. The method of claim 48 includingfurther the step of providing a control signal from the microprocessorto a digitally controlled amplifier forming part of the signalgeneration circuitry to adjust the ultrasonic signal transmitted by thefirst ultrasonic transducer.
 50. The method of claim 48 including thefurther steps of: monitoring the timing of the transmission of an otherultrasonic signal when a standard material of known acoustic propertiesis placed between the first and second ultrasonic transducers; andnumerically analyzing the second digitized signal representing the otherultrasonic signal as affected by the imposition of the standardmaterial, to measure a time of transmission of the ultrasonic signalthrough the standard material between the first and second transducersto compare the measures of time through the human heel and the measuresof time through the standard material.
 51. The method of claim 48further including the step of numerically analyzing the digitized signalto measure a change in shape of the waveform.
 52. The method of claim 51wherein the step of numerical analysis measures the change in shape ofthe waveform with respect to a standard waveform.
 53. The method ofclaim 52 wherein the step of measuring the change in shape of thewaveform provides an indication of attenuation of specific frequencycomponents through the human heel.
 54. The method of claim 48 includingthe steps of: generating a second transducer signal under the control ofa microprocessor; communicating the second transducer signal to thefirst ultrasonic transducer to produce a second ultrasonic signaldirected along a transmission axis through a human heel; receiving thesecond ultrasonic signal after distortion by the human heel at thesecond ultrasonic transducer; receiving a second electrical signal fromthe second ultrasonic transducer with an analog to digital converter toproduce a second digitized signal; and numerically analyzing the seconddigitized signal of the second ultrasonic signal to measure a change inshape of the waveform.
 55. The method of claim 48 including further thesteps of: repeating steps (a)-(d) of claim 48 and including theadditional step of: digitally averaging a plurality of digitized signalsproduced by each repetition, as affected by the imposition of a humanheel between the first and second ultrasonic transducer, to produce acomposite waveform for numeric analyses.
 56. The method of claim 48further including the step of displaying a physical property of the oscalcis of the heel based on the numerical analysis.
 57. A self-containedultrasonic instrument for measurement of the human os calcis in vivocomprising: a housing having mounting brackets holding a first andsecond ultrasonic transducer in opposition along an axis of acousticpropagation across a gap sized to accept a human heel; signal-processingcircuitry electrically connected to the first and second ultrasonictransducers to provide an ultrasonic signal to the first transducer andto receive an attenuated ultrasonic signal from the second ultrasonictransducer; a microprocessor contained within the housing and receivinga digitized form of the attenuated ultrasonic signal from the signalprocessing circuitry to compute a real-time measure of bone quality; andan electronic display communicating with the microprocessor to receivethe real time measure of bone quality displaying of the measure of bonequality to a user in real time.
 58. The self-contained ultrasonicinstrument of claim 57 wherein the digital display is supported on thehousing for real-time viewing by an operator of the instrument duringthe measurement process.
 59. The ultrasonic instrument of claim 57wherein the microprocessor executes a stored program to compute ameasure of indicating sound speed.
 60. The ultrasonic instrument ofclaim 57 wherein the microprocessor executes a stored program tocomputer a measure of indicating change in shape between the transmittedultrasonic signal and the received ultrasonic electrical ultrasonicsignal.
 61. The ultrasonic instrument of claim 57 wherein themicroprocessor executes a stored program to compute a measure ofindicating sound speed and a measure of indicating change in shape ofthe transmitted ultrasonic signal after passing through the human heel.62. A method of measurement of the human os calcis in vivo comprisingthe steps of: (a) positioning a human foot within a housing havingmounting brackets holding first and second ultrasonic transducers inopposition along an axis of acoustic propagation across a gap sized toaccept a human heel; (b) provide an ultrasonic transmission electricalultrasonic signal to the first transducer and to receive an attenuatedultrasonic signal from the second ultrasonic transducer; (c)transmitting a digitized form of the attenuated ultrasonic signal to amicroprocessor contained within the housing; (d) compute a real-timemeasure of bone quality; and (e) display in real time the measure ofbone quality to an operator whereby the operator may modify themeasurement process based on the display.
 63. The method of claim 62including the step of supporting the display on the housing forreal-time viewing by an operator of the instrument during themeasurement process.
 64. The method of claim 62 wherein step (d)displays a measure indicating sound speed.
 65. The method of claim 62wherein step (d) displays a measure indicating BUA.
 66. The method ofclaim 65 wherein step (d) displays a measure indicating speed of sound.